Stenella coeruleoalba (Mediterranean subpopulation) 

Scope: Global & Mediterranean
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Chordata Mammalia Cetartiodactyla Delphinidae

Scientific Name: Stenella coeruleoalba (Mediterranean subpopulation)
Parent Species:

Assessment Information [top]

Red List Category & Criteria: Vulnerable A2bcde ver 3.1
Year Published: 2012
Date Assessed: 2010-08-09
Assessor(s): Aguilar, A. & Gaspari, S.
Reviewer(s): Notarbartolo di Sciara, G. & Perrin , W.

This subpopulation of Striped Dolphin has been, and is currently, subject to a number of threats that, cumulatively, reduced its size and may still be slowing its recovery. An abrupt reduction in population size occurred in 1990–1992 due to a die-off involving many 1,000s of deaths (Bortolotto et al. 1992, Aguilar and Raga 1993). Immediately after this die-off, the mean school size was found to be less than one third of prior levels, which implied a proportional reduction in overall population size (Forcada et al. 1994). In 2001–2003, the density (0.49 dolphin/km²) of Striped Dolphins estimated in the Gulf of Valencia was again close to the maximum reported for this species in the western Mediterranean (Gómez de Segura et al., 2006). No similar information is available from other areas. In a 17-year study of variability in group size, Gaspari et al. (in prep.) have found a gradual increase over time in the Ligurian Sea.

The primary cause of the die-off was a morbillivirus infection (Domingo et al. 1990). PCBs and other organochlorine pollutants with potential for immunosuppressive effects were suggested to have enhanced its lethality (Aguilar and Borrell 1994). In 2006–2007 another morbillivirus outbreak affected the species off the coasts of Spain (Fernández et al. 2008, Raga et al. 2008) and later extended to France and the Italian Ligurian Sea (Garibaldi et al. 2008). The mortality associated with that event seemed more moderate (though at least 200 carcasses were reported in Spain), probably because part of the adult population still preserved antibodies against morbillivirus (Van Bressem et al. 2001, Raga et al. 2008). The reduction of the organochlorine load in the population resulting from a general decline in pollutant levels in the Mediterranean (Aguilar and Borrell 2005) may also have implied a reduction in mortality (Castrillon et al. in press).

Besides their immunosuppressive action, PCBs and other organochlorines impair reproduction through a variety of mechanisms. Past and current levels much exceed thresholds from which reproductive effects have been observed in Bottlenose and other dolphins. PCBs may have played a role in the occurrence of luteinized cysts in the ovaries of four of 56 (7.1%) Mediterranean Striped Dolphins (Munson et al. 1998) although there is no evidence that these cysts have impaired reproduction to such an extent that it would affect the overall population.

Incidental capture in fishing gear, particularly in pelagic driftnets, has been high in at least the last two decades, causing thousands of deaths per year (Silvani et al. 1999, Imbert et al. 2001, Tudela et al. 2003). Finally, given the overexploited state of most fishing resources in the Mediterranean, decreased food availability caused by fishing may be an added limitation on the recovery  of the Striped Dolphin population, although the significance of such a potential threat has not been assessed.

It is suspected that a reduction in population size of >30% occurred over the last three generations (ca. 60 years) (Criterion A2) based on the 2/3 reduction in mean school size in the 1990s (here considered an index of abundance) (A2b), a decline in quality of habitat, particularly food availability (A2c), past and current high levels of exploitation in the form of incidental mortality in fisheries (A2d), and the effects of pathogens and pollutants (A2e).

The causes of the population reduction may be reversible but are neither clearly reversible nor understood; moreover, none of the threats (i.e. potential or likely causes of the decline) has ceased except for pollution by organochlorine pollutants, which has declined in the last two decades (Aguilar and Borrell 2005). Reducing the health threats still caused by PCBs and others pollutants such as brominated flame retardants, polycyclic aromatic hydrocarbons and perfluorinated compounds may require that these pollutants decrease to a non-effect threshold.


Geographic Range [top]

Range Description:This species occurs in temperate and subtropical waters of all oceans. It is present throughout the Mediterranean Sea but has not been recorded in the Black Sea.
Countries occurrence:
Algeria; Croatia; Cyprus; Egypt; France; Gibraltar; Greece; Israel; Italy; Malta; Monaco; Morocco; Spain; Tunisia; Turkey
FAO Marine Fishing Areas:
Mediterranean and Black Sea
Additional data:
Range Map:Click here to open the map viewer and explore range.

Population [top]


Population structure
Morphological and genetic studies strongly suggest that the Mediterranean and eastern North Atlantic populations are genetically differentiated, with little or no gene flow across the Straits of Gibraltar. Maximum body length of eastern North Atlantic striped dolphins is 5-8 cm longer than that of Mediterranean individuals (Calzada and Aguilar 1995). Skull size is also smaller in Mediterranean specimens than in their neighbouring Atlantic counterparts (Archer 1997). Mitochondrial DNA analysis, from Gibraltar to Greece, yielded 59 haplotypes (n=166), none of which was shared between the two areas, thus supporting strong differentiation; analyses of nuclear DNA support this result (Gaspari et al. 2007, Gaspari et al. in prep).

Subpopulation structure
Genetic analyses of Striped Dolphins from Gibraltar to the coast of Israel, both at nuclear and mitochondrial levels, show that this species is genetically structured within the Mediterranean with low gene flow across the basin. Furthermore, there is evidence for sex-biased dispersal (Gaspari et al. in prep). On a smaller geographic level, Gaspari (2004) found evidence of genetic differentiation between inshore and offshore populations in the Ligurian Sea. Moreover, inside the Mediterranean there is some clinal variation in body size suggestive of population structure and/or restriction in gene flow between areas (Calzada and Aguilar 1995). This appears to be confirmed by significant differences in tissue pollutant levels between Spain and Italy (Monaci et al. 1998).

Range and population
Although overall the Striped Dolphin is the most abundant cetacean in the Mediterranean, both in the eastern and the western basins, it is not found at uniform densities. It typically shows a preference for highly productive, open waters beyond the continental shelf (Notarbartolo di Sciara et al. 1993, Forcada et al. 1994, Frantzis et al. 2003, Gannier 2005). A small number of Striped Dolphins may be resident in the eastern portion of the Gulf of Corinth (Greece) (Frantzis and Herzing 2002). Two strandings were recorded in the Marmara Sea in 1990s (Öztürk et al. 1999).

Reliable abundance estimates are only available for the western basin and most of them refer to the period immediately or soon after the 1990–1992 die-off. There is no available estimate for the eastern Mediterranean Sea:
  • Western Mediterranean excluding the Tyrrhenian Sea (1991): 117,880  [95% CI = 68,379–214,800] (Forcada et al. 1994).
  • Balearic Sea (1991): 5,826 [95% CI = 2,193–15,476] (Forcada and Hammond 1998).
  • Gulf of Lions (1991): 30,774 [95% CI = 17,433-54,323] (Forcada and Hammond 1998).
  • Ligurian Sea (1992): 14,003 [95% CI = 6,305–31,101] (Forcada et al. 1995).
  • South Balearic area (1991): 18,810 [95% CI = 8,825–35,940] (Forcada and Hammond 1998).
  • Alboran Sea (1991): 17,728 [95% CI = 9,507–33,059] (Forcada and Hammond 1998).
  • Central coast of Spain (2000–2002): 15,778 [95% CI = 10,940–22,756] (Gómez de Segura et al. 2006).
Population trend
the population declined in the early 1990s. Current population trend is unknown, although the population may have recovered to pre-1990 levels, at least in some areas (Gómez de Segura et al. 2006).
Current Population Trend:Unknown
Additional data:

Habitat and Ecology [top]

Habitat and Ecology:The Striped Dolphin is an oceanic species. It shows a preference for highly productive, open waters beyond the continental shelf. It is particularly abundant in the Ligurian Sea, the Gulf of Lions, the waters between the Balearic Islands and the Iberian Peninsula, and the Alborán Sea. In the Ligurian Sea, this species showed no preference for any specific physiographic features, but was almost homogeneously distributed (Azzellino et al. 2008).

Life History Parameters
Age at maturity:
Females: 12 years (western Mediterranean population) (Calzada et al. 1996)
Males: 11.3 years (western Mediterranean population) (Calzada 1996)

Females: 32 years (Di Mèglio and Romero-Álvarez 1996, Calzada et al. 1997)
Males: 28 years (Di Mèglio and Romero-Álvarez 1996, Calzada et al. 1997)

Percentage of living population that is reproductively mature:

Average age of parents in the population:
Females: 22 years (Calzada et al. 1996, Calzada et al. 1997)
Males: 20 years (Calzada et al. 1996)

Gestation time:
Slightly over 12 months (Aguilar 1991)

Average interbirth interval:
2.7 years (in Japan) (Kasuya 1985)

Maximum potential annual rate of population increase:
0.09 (Schmitz and Lavigne 1984)

Use and Trade [top]

Use and Trade:

In the past this species was hunted for use as bait for shrimp traps and longlines. Despite being forbidden, catches with this aim continue in at least southern Spain and probably other areas. Certain number of direct catches for human consumption or for use as bait, still continue in several Mediterranean countries (SGFEN 2001).

Threats [top]

Major Threat(s):

In 1990–1992 a die-off devastated much of, if not the entire, Mediterranean population; 1,000 carcasses were examined in Spain, Italy and France alone, but the toll was undoubtedly much higher because these countries represent only a fraction of the Mediterranean coastline known to have been affected by the process (Bortolotto et al. 1992, Aguilar and Raga 1993). Immediately after the event, the mean school size was found to be less than one third of original levels, which may be taken as an indication, but not as a proof, for a proportional reduction in overall population size (Forcada et al. 1994). The primary cause of the die-off was a morbillivirus infection (Domingo et al. 1990, Van Bressem et al. 1993). The epidemic started in regions containing unusually large numbers of inbred dolphins that were possibly more susceptible to diseases (Valsecchi et al. 2004). PCBs and other organochlorine pollutants with potential for causing immunosuppressive effects were suggested to have enhanced its lethality because the individuals that succumbed to the disease were those carrying the highest PCB tissue concentrations (Aguilar and Borrell 1994).

A second outbreak occurred in 2006–2007 affecting the coasts of Spain (Fernández et al. 2008, Raga et al. 2008), France and the Italian Ligurian Sea (Garibaldi et al. 2008). The mortality associated with that event was moderate, with only about 200 carcasses reported, probably because older dolphins were still protected by the immunity developed during the 1990–1992 epidemic (Raga et al. 2008). Moreover, differently to the previous outbreak, PCB levels in the individuals that succumbed to this event were not significantly different from those in the surviving population, which suggests that pollutants were of no relevance to the 2006–2007 event (Castrillon et al. in press). Recurrent epidemics may have profound cumulative effects on the population dynamics of Mediterranean Striped Dolphins (Van Bressem et al. 2009a).

Poxviruses with the potential to cause mortality of neonates and young calves are also circulating in this population (Van Bressem et al. 2009b). Marine brucellae also infect Mediterranean Striped Dolphins (Van Bressem et al. 2001). These bacteria may limit recruitment by compromising the normal functioning of male and female reproductive systems, inducing abortions and killing neonates and sexually mature individuals (Van Bressem et al. 2009a). Finally, infection by the protozoan Toxoplasma gondii seems to be common in Mediterranean striped dolphins (Domingo et al. 1992, Cabezon et al. 2004) and has been suggested as another factor contributing to the death toll during morbillivirus epidemics.

Tissue levels of organochlorine compounds, some heavy metals, selenium and possibly other pollutants such as the brominated flame retardants, polycyclic aromatic hydrocarbons and perfluorinated compounds are high and often exceed threshold levels above which detrimental effects commonly appear in mammals (Aguilar 2000, Marsili et al. 2001, Kannan et al. 2002, Pettersson et al. 2004). Blubber concentrations of DDT and PCB, the two main organochlorine pollutants, show a slowly declining trend in the last two decades (Aguilar and Borrell 2005) but are currently still high. Besides the alleged immunodepressive effect (see above), high PCB levels may have been implicated in the development of unusual luteinized cysts in the ovaries of four of 56 (7.1%) Mediterranean Striped Dolphins (Munson et al. 1988), although other factors like the morbillivirus infection, stress and food depletion could have contributed to the abnormal surge or release of luteinizing hormone.

The Spanish driftnet fishery in the Alboran Sea reportedly killed 148–170 dolphins per season in the early 1990s (Silvani et al. 1999); this fishery was halted in 1995 but the nets were transferred to Moroccan boats, which continue operating and are estimated to kill in the order of 1,500–2,000 Striped Dolphins per year (Tudela et al. 2003). The Italian driftnet fishery (spadare fishery) has been claimed to kill 5,000–15,000 dolphins, mostly Striped Dolphins, per year (Di Natale 1992); although fishing effort is declining, current catch levels are thought to be still high. The French thonaille driftnet fishery has been estimated to kill 180–472 Striped Dolphins per season (Imbert et al. 2001). Reports from other fisheries are sparse and bycatch data are not collected systematically, but what evidence there is suggests that incidental catches of Striped Dolphins are widespread and likely to represent a significant toll at least in pelagic purse-seines, drifting long-lines and gill nets (Di Natale and Notarbartolo di Sciara 1994). To this should be added a certain number of direct catches for human consumption or for use as bait, which still continue in several Mediterranean countries (SGFEN 2001).

The diet of Striped Dolphins includes commercial fish and cephalopod species (Pulcini et al. 1992, Blanco et al. 1995), so the widespread depletion of fishery resources in the Mediterranean has the potential to affect Striped Dolphin numbers.

Global warming may have significant direct and indirect effects on cetacean populations in the Mediterranean (Gambaiani et al. 2009). Climate variability and change affect biomass in a number of ways, including shifts in species distribution. Azzellino et al. (2008b) showed a direct effect of sea surface temperature on striped dolphin distribution in the Ligurian Sea.

Conservation Actions [top]

Conservation Actions:

No specific measures have been taken for the conservation of Striped Dolphins in the Mediterranean Sea, although this species benefits from the generic protection laws existing in many range states for cetaceans.

One area where the species is more abundant is protected (in principle at least) under the Marine Sanctuary for Cetaceans in the Corso-Ligurian Basin, declared by the Governments of Italy, France and Monaco. Apart from this, because the Striped Dolphin is an oceanic species, most currently existing protected areas are of no use for its conservation. Development of offshore protected areas through international agreements similar to the case in the Ligurian sea should be encouraged. In Spain there was a governmental initiative to identify areas of special interest for the conservation of cetaceans, but the recommendations put forward have not materialized. Effective protection of these areas should be enforced and similar initiatives should be undertaken in other Mediterranean countries.

Pelagic driftnets have been forbidden in Spain and are limited by UE regulations. However, an Italian fleet of significant size (ca 100 vessels) still fishes with driftnets in an unregulated manner. To this is should be added a large Moroccan fleet and the French thonnaille vessels, all know to be responsible for significant cetacean mortality. Drift nets should be eliminated from the region or, at a minimum, the existing regulations on that gear should be strictly enforced.

Most Mediterranean countries have regulations prohibiting direct takes, but enforcement is sometimes poor. This should be corrected and deliberate killing stopped.

Control of pollution, particularly that by organochlorine compounds, has become more effective in the last two decades and the levels of those pollutants are decreasing. However, existing laws and control should be further enforced and Striped Dolphin populations should be monitored to assess trends and geographical variation of known pollutants in tissue levels.

The population size was estimated in the western Mediterranean immediately after the 1990 die-off. Abundance should be monitored, particularly to assess recovery from the die-off(s).

Occurrence of ovarian cysts should be monitored in the population and their potential impact on reproduction should be investigated.

Diet should be studied through stomach content and isotopic analyses to assess overlap with commercial fisheries.

Stranded dolphins should be examined for viral, bacterial and Toxoplasma gondii infection and for the presence of antibodies against these agents.

Citation: Aguilar, A. & Gaspari, S. 2012. Stenella coeruleoalba (Mediterranean subpopulation). The IUCN Red List of Threatened Species 2012: e.T16674437A16674052. . Downloaded on 19 August 2018.
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